The knee comprises three inter-dependent joints in three separate compartments, all surrounded by a fibrous capsule covered by the skin. The medial tibio-femoral joint involves contact between the thigh bone (the femur) and the leg bone (the tibia) on the inside of the lower limb. The lateral tibio-femoral joint involves contact between the femur and the tibia on the outside of the lower limb. The patello-femoral joint involves contact between the femur and the knee cap (the patella) on the front of the lower limb.
The front of the lower (distal) end of the femur comprises a flanged concave groove providing a track for the patella. The back of the distal femur divides into two separate near-spherical concave condyles making contact with the tibia. The upper surface of the tibia is like a plateau which is slightly dished on the medial side for contact with the medial femoral condyle forming the medial tibio-femoral joint and slightly convex on the lateral side for contact with the lateral femoral condyle forming the lateral tibio-femoral joint with a protrusion (the tibial eminence) running from front to back between the joints.
The articulating surfaces in each joint are covered with thin layers of a tough protective layer called cartilage, and are lubricated by synovial fluid secreted from a membrane on the inner surface of the fibrous capsule surrounding the knee. The surfaces of the tibio-femoral joints are further separated by the menisci, semi-circular semi-lunar collagen bundles oriented circumferentially. Each bundle is securely attached at each end to the tibia. The menisci form closely-fitting sockets for the femoral condyles bringing the dissimilar surfaces of the femur and tibia into closer conformity.
The bones are held together actively by muscles with their tendons which span the joints and passively by ligaments and the joint capsule. The ligaments comprise bundles of collagen fibres running mainly longitudinally. The collateral ligaments arise on the external surfaces of the medial and lateral condyles. The medial collateral ligament inserts into the external medial surface of the proximal tibia. The lateral collateral ligament inserts into the proximal surface of the fibula. The medial collateral ligament is a much larger and stiffer structure than the lateral collateral ligament. The cruciate ligaments arise from the internal surfaces of the femoral condyles and insert into the tibial eminence.
The ligaments and the bones together form a mechanism which controls a complex pattern of movement of the bones on each other. In the unloaded state, flexion of the knee to 130° about a transverse axis is accompanied by approximately 25° rotation about the axis of the tibia (axial rotation) and approximately 5° about an anteroposterior axis (abduction-adduction). These movements are accommodated by mainly antero-posterior translations of the tibio-femoral contact areas so that the bones roll as well as slide on each other and the patella slides over the anterior femur.
Under load, the ligaments stretch and the articular surfaces indent, significantly modifying the relationship between flexion, axial rotation and abduction-adduction and between flexion and contact area translations. Movements at the knee are therefore load and activity dependent.
Damage to the articular surfaces or to the ligaments changes the patterns of movement of the bones on each other and response of the joint to load. Osteoarthritis follows from failure of the cartilage in one or other of the three joints, leading to bone-on-bone contact and the onset of pain. Frequently, osteoarthritis first manifests itself in the medial compartment, while the ligaments remain intact. The disease can remain confined to the medial compartment until the anterior cruciate ligament fails and the disease then spreads to the other two compartments. No drug treatment has been found which reverses these processes.
Total knee replacement is the most common surgical treatment for osteoarthritis, involving replacement of the articular surfaces of all three compartments and sacrifice of some of the ligaments. Partial knee replacement involves replacement of the articular surfaces in only one compartment, leaving intact the surfaces of the other two compartments and all of the ligaments. Partial knee replacement can act prophylactically, reducing the rate of development of the disease in the other compartments. Partial knee replacement is surgically more demanding and is not always used when it is indicated.
To implant the prosthetic components of a knee replacement, sufficient sections of bone first have to be removed from the surfaces of the tibia and the femur. The tibial surface is sawn first, using a saw guide that is applied with its shaft parallel to the long axis of the tibia and its upper end nailed to the exposed bone. The level of the guide is estimated, usually consarvatively so that more bone can be resected if required.
Once the initial resection of the tibia has taken place, the femur is prepared for the implantation of the femoral component of the knee replacement. Holes are drilled for insertion of the femoral saw block, the spigots for milling and the trial femoral component. The positions and angles of these holes are referenced by inspection off features of the femur, and the position of the resected tibial plateau is checked and revised if necessary. Once the initial cutting, milling and trimming of the femur has taken place, a trial femoral component and a tibial template are inserted. The new gap between the femoral and tibial components is measured at the extremes of knee flexion, and the femur is further milled to equalise these flexion gaps. Final trimmings of the femur and tibia take place before the appropriate bearing is fitted and the final components are cemented in place.